1. Field of the Invention
The present invention relates to a poppet valve for the intake and exhaust valves of an internal combustion engine and also relates to a method of producing same.
2. Description of the Prior Art
The poppet valve which is used as the exhaust valve of an internal combustion engine is exposed to corrosive exhaust gases under a high temperature condition such as above 800.degree. C and, furthermore, it is subjected to strong tensile and bending stresses at its neck portion due to the spring force applied by the valve spring and its own inertia when it abuts against the valve seat. In addition to the oxydizing action effected by C02, H20, residual O2, etc., contained in the hot exhaust gas, particularly in the gasoline engine, the exhaust poppet valve is effected by the hot corrosion due to the combustion products generated from lead compounds added to gasoline as an anti-knock substance. Furthermore, the face portion of the poppet valve undergoes friction wear due to frictional contact with the valve seat. Since the abovementioned various effects applied to the poppet valve become more serve as the output power and the rotational speed of the engine increase, there is a demand for even more improved poppet valves to improve the reliability of the engine and the design of a high speed engine.
To meet with this demand, a poppet valve having a stellite outer layer provided by melt-moulding is being used more and more used for the exhaust valve. However, the melt-moulding requires considerable skill and a complicated manufacturing process, whereby the manufacturing cost of this type of valve is relatively high resulting in an economical disadvantage. Also, this type of valve does not have sufficient durability to serve as an exhaust valve for modern high speed engines or engines designed for exhaust gas purification. For the use of the unleaded gasoline proposed in recent years, a valve seat insert is often used as a countermeasure therefor. When the valve seat insert is employed, the temperature of the exhaust poppet valve becomes higher than in the conventional case, whereby it is required that the high temperature strength and the resistance to high temperature corrosion properties of the poppet valve are improved. On the other hand, in those countries where high lead gasoline is still used, the exhaust poppet valve is required to have a high resistance to the lead compounds such as lead oxide, etc.
One of the materials that is at present most widely used for the exhaust poppet valve of the internal combustion engine is a valve steel having the composition: Fe--0.5C--21Cr--4Ni--9Mn--0.4N. However, for use under the severe operating conditions as encountered in the modern high performance engine, it is required that the valve, particularly the head portion thereof has a high temperature strength and resistance to oxygen, lead oxide etc. which are of a higher degree as compared with the abovementioned conventional valve steel.
As a super alloy material to meet with this severe demand, high temperature corrosion-resistant nickel base alloys have been proposed in Japaneses patent application No. 1671/74 filed by the same applicant as the applicant of the basic Japanese application of the present application. These alloys are characterized by the following composition: less than 0.15% C, 15-26% Cr, 1-10% Co, 5-18% Fe, more than 0.8% Ti, more than 0.8% Al, provided that Ti plus Al is between 1.6-5.0% and balance Ni and impurities, or less than 0.15% C, 15-26% Cr, 1-10% Co, 5-18% Fe, more than 0.8% Ti, more than 0.8% Al, provided that Ti plus Al is between 1.6-5.0%, less than 2% either Mo or W or both, and balance Ni and impurities.
Furthermore, according to our recent development, a super alloy having the composition: less than 0.05% C, 19.0-21.0% Cr, 1.0-3.0% Co, 2.5-3.0% Ti, 1.2-1.6% Al, 0.3-5% Nb +Ta, 11.0-13.0% Fe and balance Ni and impurities, has been found to particularly meet the severe operating conditions of the exhaust valve. This super alloy is advantageously upset-forged at a temperature of 1060-1130.degree. C.
By employing these super alloys as the material for forming an exhaust poppet valve, it is possible to obtain a valve which has excellent high temperature strength and resistance to oxidization and corrosion by lead oxide, etc. However, since these super alloys are very expensive, a high cost is incurred if the entire body of the poppet valve is formed of these super alloys.